On 28-Nov-00 The Hermit Hacker wrote:
> Just tried to build a kernel based on sources from today, to enable
> BREAK_TO_DEBUGGER so that I can try and get in and see where its hanging
> ... the compile hung the machine solid.  Even hitting the
> 'numlock'/'capslock' on my keyboard generated no results ...

It is spinning with interrupts disabled, probably due to holding a spinlock for
far too long.  Debugging this is not all that fun.  :-P  If you can rig up an
NMI switch, you can use that to drop into ddb and then use 'x' to see who owns
various mutexes (sched_lock and callout_mtx being the primary spin mutexes of
concern).  If you compile your kernel with WITNESS and MUTEX_DEBUG, then you
can use 'x' to look at the sched_lock and callout_mtx mutex structures, find
the pointer to the mtx_debug structure, and examine that to find the mtxd_file
and mtxd_line members.  Then you can look at those (x/s to look at the filename
as a string) to find the filename and line number when the mutex was last
acquired.  Grr, except that this is broken for spin mutexes.  If you are
patient, you can try rigging up a serial console, compile KTR into your kernel
as so:

options KTR
options KTR_EXTEND

Then when the machine has booted, log in via ssh or a tty other than the serial
console and type the following:

# sysctl -w debug.ktr_mask=0x1208
# sysctl -w debug.ktr_verbose=2
# while (1) do
> make -j 16 buildworld
> end

Unfortunately, there is a chance the machine will die before it hangs due to
exceeding the stack space.  In that case, you can _try_ bumping UPAGES, but
that didn't help on my test machines. :-/  However, if your machine doesn't
blow up and die, then when it hangs, the KTR output dumped to the serial console
(which you should probably log to a file via script or somesuch) will show what
mutex was acquired and where it was acquired that is causing the hang.


John Baldwin <[EMAIL PROTECTED]> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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